Rubber Elasticity, Chemistry tutorial

Introduction

Natural rubber, as well said China rubber or caoutchouc, is an elastomer (an elastic hydrocarbon polymer) that was originally derived from latex, a milky colloid produced by some plants. The purified form of natural rubber is the chemical polyisoprene, which can also be produced synthetically. Natural rubber is utilized broadly in many applications and products, as is synthetic rubber. It is usually extremely stretchy and flexible and extremely waterproof.

Rubber Elasticity is described as ability of rubber to exhibit extremely huge deformability through complete recoverability. Rubber elasticity can be quantitatively compared to the behaviour of a metallic spring. When we apply several forces to a spring, it elongates via virtue of its uncoiling and when we release the force, the spring regains its original shape owing to recoiling. A similar thing occurs in the case of rubber band also. The chain molecules undergo uncoiling and recoiling on the application and release of a small force. In order to undergo coiling and recoiling, the molecular chains in the rubber band should comprise flexible segments capable of attaining free rotation.

In the unstrained state, they should tend to take up the more probable randomly coiled conformation such that the entropy factor is the highest at its normal state. When strained, the chains should be able to get extended and be brought to more ordered conformations. An ordered arrangement of the chain molecules gives rise to partial crystallinity and also a decrease in the entropy factor. It is then the entropy factor that favors recoiling on release of the force. Therefore, rubber exhibits unique physical and chemical properties. Its stress-strain behavior exhibits the Mullins consequence, the Payne effect, and is frequently modeled ashyperelastic. Rubber strain crystallizes. Owing to the occurrence of a double bond in each repeat unit, natural rubber is sensitive to ozone cracking.

Natural rubber 

Latex is a natural polymer of isoprene (most often cis-1,4-polyisoprene) - with a molecular weight of 100,000 to 1,000,000. Typically, a small percentage (up to 5% of dry mass) of other materials, such as proteins, fatty acids, resins and inorganic materials (salts) are found in natural rubber. Polyisoprene is as well generated synthetically, producing what is sometimes termed to as 'synthetic natural rubber'.

Some natural rubber sources said gutta-percha are composed of trans-1, 4-polyisoprene, a structural isomer that has similar, but not identical, properties. Natural rubber is an elastomer and a thermoplastic. Though, it should be noted that once the rubber is vulcanized, it will turn into a thermoset. Most rubber in everyday utilize is vulcanized to a point where it splits properties of both; for example, if it is heated and cooled, it is degraded but not demolished.

Elasticity of Rubber

In most elastic substances such as metals utilized in springs, the elastic behaviour is caused through bond distortions. When force is applied, bond lengths deviate from the (minimum energy) equilibrium and strain energy is stored electrostatically. Rubber is frequently assumed to behave in the similar way, but it turns out this is a poor description. Rubber is a curious substance since, unlike metals, strain energy is stored thermally. When rubber is stretched, the 'loose pieces of rope' are taut and therefore no longer able to oscillate. Their kinetic energy is specified off as excess heat.

Therefore, the entropy reduces when going from the relaxed to the stretched state, and it enhances during relaxation. This transform in entropy can as well be described through the fact that a tight section of chain can fold in fewer ways (W) than a loose section of chain, at a specified temperature (note: entropy is defined as S=k*ln(W)). Relaxation of a stretched rubber band is therefore driven via enhance in entropy, and the force practiced isn't electrostatic, rather it is a consequence of the thermal energy of the substance being transferred to kinetic energy.

Thermodynamics of Rubber Elasticity 

The 1st and 2nd law of thermodynamics applied to a reversible, equilibrium procedure provides the relationship between internal energy (U), entropy (S), and work (W) as  

                                       dU = TdS - dW                              

For deformation of a rubber band, the work is an amalgamation of pressure-volume development and the work due to the (tensile) force (f) applied to the rubber band, specified as 

                                      dW = pdV - ƒdl                               

Where the convention is that work done via the system (for instance, pressure-volume work) is positive and the work done on the system (for instance, force-displacement work) is negative. Enthalpy is related to the internal energy as 

                                      dH = dU + pdV.                               

Substituting eqtns and (in the form pdV = dW + ƒdl) into eqtn. gives

                                     dH = TdS + ƒdl.                                

At steady temperature and pressure, rearrangement of eqtn in terms of force and taking the (partial) derivative through respect to length at constant temperature and pressure provides

ƒ =   (δH/ δl) T,p    -   T (δS/ δl) T,p.

Equation illustrates that elastic force has both an enthalpic and entropic component (ƒ = ƒe + ƒs). In order to attain an expression for the temperature dependence of force, we build utilize of the relation

                                    (δS/ δl) T,p   =   -   (δƒ/ δT ) p, l.                

Substituting above eqtn into eqtn gives

                           ƒ = (δH/ δl)T,p   +  (δƒ/ δT) p, l                   

Main uses of Natural Rubber

Because of its elastiscity, resilience and toughness, natural rubber (NR) is the essential constituent of many products utilized in the transportation, industrial, consumer, hygienic and medical sectors. Evaluated to vulcanized rubber, uncured rubber has comparatively few utilizes. It is employed for cements, for adhesive, insulating, and friction tapes; and for crepe rubber used in insulating blankets and footwear. Vulcanized rubber, on the other hand, has abundant applications. Resistance to abrasion makes softer kinds of rubber valuable for the treads of vehicle tyres and conveyor belts, and builds hard rubber valuable for pump housings and piping utilized in the handling of abrasive sludge.

The flexibility of rubber is frequently utilized in hose, tires, and rollers for a broad diversity of machines ranging from domestic clothes wringers to printing presses; its elasticity builds it appropriate for diverse kinds of shock absorbers and for specialized machinery mountings designed to reduce vibration. Being comparatively impermeable to gases, rubber is helpful in the manufacture of articles such as air hoses, balloons, balls, and cushions. The resistance of rubber to water and to the action of most fluid chemicals has led to its use in rainwear, diving gear, and chemical and medicinal tubing, and as a lining for storage tanks, processing equipment, and railroad tank cars. Because of their electrical resistance, soft rubber goods are used as insulation and for protective gloves, shoes, and blankets; hard rubber is used for articles such as telephone housings, parts for radio sets, meters, and other electrical instruments. As well rubber is originating helpful in the medical and health sector (notably, condoms, catheters and surgical gloves) in addition to seismic materials (for instance, over 500 and 2,500 buildings are respectively fitted with seismic rubber bearings in China and Japan). The coefficient of friction of rubber, which is elevated on dry surfaces and low on wet surfaces, leads to utilize of rubber both for power-transmission belting and for water-lubricated bearings in deep-well pumps.

 Fig: Natural Rubber major end uses   

265_Natural Rubber major end uses.jpg

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